1. Field of the Invention
The invention is a method of forming a multiple layer dielectric and a hot-film sensor formed therewith, more particularly a hot-film laminar flow separation sensor for use in continuous, high pressure cryogenic wind tunnels under high Reynolds numbers such as the National Transonic Facility (NTF) at Langley Research Center, Hampton, Va.
2. Description of the Prior Art
Various types of hot film sensors for use in ambient temperature subsonic and transonic wind tunnels and in cryogenic facilities are known. Hot-film sensors are generally preferred over hot-wire, surface pitot tubes and other known systems for detecting the beginning and end of boundary layer transition on models tested in conventional (near ambient) temperature and pressure wind tunnels.
However, conventional hot-film sensors of the quartz plug type are unsuited for use in cryogenic wind tunnels because of the wide range of temperatures (ambient to cryogenic) and the high Reynolds numbers (i.e., 3.0-10.5 million) encountered during testing.
This is because the wide range of tunnel temperature causes a differential growth between the hot-film installation and the electrically conductive surface of the test model to which attached. This differential growth can lead to surface roughness that exceeds the critical roughness height at the normal high unit Reynolds numbers developed in continuous high pressure cryogenic tunnels such as the NTF causing an undesirable and misleading boundary layer transition.
It is frequently required to test models having a wing span of four to five feet. Such tests can require one hundred or more hot-film laminar separation sensors to adequately locate the boundary layer transition on-line over a wide range of model attitudes (i.e., angle of attack and yaw) and test conditions (i.e., Mach number and Reynolds number) that are encountered in a typical force and moment test in a wind tunnel such as the NTF.
Conventional hot-film sensors formed by the vapor deposition of a metallic and resistive film and gold connecting leads upon a dielectric substrate such as an epoxy paint applied to the metal surface of the wing to be tested have not proved successful for use in a continuous high pressure cryogenic environment. One reason is that the resulting thickness of such hot-film sensors is frequently in excess of 0.006 inches which exceeds the permissible critical roughness height. Due to the large surface area required to form such hot-film sensors, center to center spacing is greater than 0.125 inches.
In addition, such epoxy paint substrates are extremely difficult to smooth down in thickness without effecting the surrounding surface area of the model to which attached; have a tendency to fail at cryogenic temperatures; and are not of sufficient flexibility for use on two and three dimensional surfaces as found on many test models.
Other systems for sensing laminar separation using flow visualization, infrared thermography, and fluctuating surface pressures are known, but have been found either intrusive or inadequate for use in NTF operating conditions.